Flow-volume curves, conventional lung function tests, are widely used to measure airflow limitation, and hence the degree of bronchial obstruction of small bronchial airways in children and adults with lung diseases.
The upper trace in FIG. 1 of the accompanying drawings is a flow-volume curve produced by asking the subject to breathe in as much as possible until total lung capacity (TLC) is reached, and then to breathe out as quickly and as strongly as possible. The trace shows on the axes of abscisses changes in volume, measured from the total lung capacity (TLC) to residual volume (RV), the difference between them being defined as the vital capacity (VC). The ordinate indicates the changes in flow. In the first part of the curve, a high flow, the so called peak expiratory flow (PF) or "peak flow" is reached. Thereafter, the volume is displaced and the flow decreases in a healthy subject linearly from peak expiratory flow (PF) to residual volume (RV). The maximal expiratory flows at 75, 50 and 25% of vital capacity (VC), defined respectively as MEF75, MEF50 and MEF25 are standardized flows with respect to a well defined lung volume.
Specific diseases which are of concern in this field include wheezy bronchitis and asthma in children, bronchial asthma, cystic fibrosis and chronic obstructive lung disease in adults (COLD). All these illnesses are characterized by a decrease in the performance of the airways, leading to bronchial obstruction which progresses with age. Functionally, bronchial obstruction is characterized by airflow limitation. There is great interest in determining the magnitude of this decrease, in order to get objective information about the degree of functional impairment of the lungs, as well as the stability of the disease. The latter, however, can only be determined if day-to-day variations in airflow limitation are followed ambulatorily by home monitoring.
There is therefore a need to have a low-cost, simple medical device for the measurement of airflow, which is independent of electric supply, easy to use, but accurate in its assessment, and which provides objective data for medical interpretation by patients and doctors.
It is standard practice for pediatricians, general practitioners and internists to measure airflow limitation on an ambulatory basis with devices such the Wright peak-flow meter or the Pulmonary Achievement Trainer, when measuring airways obstruction in patients with lung disease. The former is described in GB-A-1160669, and the latter in US-A-3826247. In such devices the maximum flow of air which a person can produce when exhaling is measured in a cylindrical tube within which a piston or a plate is actuated by the entering air and displaced from its initial zero-position to a scaled distance reflecting the maximum flow breathed through. Practical use of such devices has shown that calibration in absolute terms is difficult, intra-individual variation is high and inter-individual comparisons are rather difficult. Application is therefore limited to long-term monitoring where the patient is its one control. Application is also restricted to measurements of maximum flow of air, and hence to measurements of peak expiration flow (peak-flow). In this context it must be recognized, that peak-flow measurements only are the tip of the iceberg, so to speak, of the underlying functional disorders, reflecting only processes in large airways, whereas other obstructions are sometimes more important. In addition, blowing flow-volume curves (including that for peak-flow measurement) by a particular subject require rather unnatural forced breathing maneuvers. There is considerable scientific work showing that after repeated forced breathing maneuvers test-induced bronchospasms can occur. In addition, especially in children and older patients considerable difficulties are encountered in blowing accurately reproducible flow-volume curves because of the limited cooperation of these patients.
The description of the prior art known up to today is related to various pulmonary disease such as wheezy bronchitis, bronchial asthma, cystic fibrosis both in adults and children, and chronic obstructive lung disease (COLD) in adults requiring careful analysis of the functional condition of a patient's lungs. By requiring the patient to blow into a diagnostic device, the functional conditions of the patient's lungs can be detected. However, in the past, there has been no simple but accurate working diagnostic device for bed-side use, and hence home use, which will inform the physician with a direct read-out about the functional condition of a patient's lungs during a predetermined period of therapy.